Method for preparing anode active material

ABSTRACT

Disclosed is a method including (a) mixing a precursor of a material for preparing at least one material selected from the group consisting of low crystalline carbon and amorphous carbon with a hydrophilic material, followed by purification to prepare a mixture for coating, (b) mixing the mixture for coating with a crystalline carbon-based material to prepare a core-shell precursor in which the mixture for coating is coated on a core including a crystalline carbon-based material, and (c) calcining the core-shell precursor to carbonize the material for preparing the at least one material selected from the group consisting of low crystalline carbon and amorphous carbon into the at least one material selected from the group consisting of low crystalline carbon and amorphous carbon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT/KR2012/000101 filed on Jan. 5,2012, which claims priority under 35 U.S.C 119(a) to Patent ApplicationNo. 10-2011-0002565 filed in the Republic of Korea on Jan. 11, 2011, allof which are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a method for preparing an anode activematerial and more particularly to a method for preparing an anode activematerial comprising a core comprising a crystalline carbon-basedmaterial and a composite coating layer comprising at least one materialselected from the group consisting of low crystalline carbon andamorphous carbon and a hydrophilic material through a procedurecomprising mixing a precursor of a material for preparing the at leastone material selected from the group consisting of low crystallinecarbon and amorphous carbon with the hydrophilic material, followed bypurification to prepare a mixture for coating, mixing the mixture forcoating with the crystalline carbon-based material to prepare acore-shell precursor in which the mixture for coating is coated on thecore comprising the crystalline carbon-based material, and calcining thecore-shell precursor to carbonize the material for preparing the atleast one material selected from the group consisting of low crystallinecarbon and amorphous carbon into the at least one material selected fromthe group consisting of low crystalline carbon and amorphous carbon.

BACKGROUND ART

Technological development and increased demand for mobile devices haveled to rapid increase in the demand for secondary batteries as energysources. Among such secondary batteries, lithium secondary batterieshaving high energy density, high operating voltage, long cycle span andlow self-discharge rate are commercially available and widely used.

In addition, increased interest in environmental issues has recentlybrought about a great deal of research associated with electric vehicles(EV) and hybrid electric vehicles (HEV) as alternatives to vehiclesusing fossil fuels such as gasoline vehicles and diesel vehicles whichare main causes of air pollution. Such electric vehicles generally usenickel-metal hydride (Ni-MH) secondary batteries as power sources.However, a great deal of study associated with use of lithium secondarybatteries with high energy density, discharge voltage, and outputstability is currently underway and some are commercially available.

A lithium secondary battery has a structure in which a non-aqueouselectrolyte containing a lithium salt is impregnated into an electrodeassembly comprising a cathode and an anode, each including an activematerial coated on a current collector, with a porous separatorinterposed between the cathode and the anode.

Lithium cobalt-based oxide, lithium manganese-based oxide, lithiumnickel-based oxide, lithium composite oxide and the like are generallyused as cathode active materials of lithium secondary batteries.Carbon-based materials are generally used as anode active materials. Useof silicon compounds, sulfur compounds and the like as anode activematerials is also under consideration.

However, lithium secondary batteries have various problems, some ofwhich are associated with fabrication and operating properties of ananode.

First, regarding anode fabrication, a carbon-based material used as ananode active material is highly hydrophobic and thus has low miscibilitywith a hydrophilic solvent, thereby reducing dispersion uniformity ofsolid components, in the process of preparing a slurry for electrodefabrication. In addition, hydrophobicity of the anode active materialcomplicates impregnation of highly polar electrolytes in the batteryfabrication process. Thus, electrolyte impregnation is a bottleneck inthe battery fabrication process, greatly decreasing productivity.

Addition of a surfactant as an additive to an anode, an electrolyte orthe like has been suggested as a possible solution to the problems.However, surfactants are unsuitable due to side effects upon operatingproperties of batteries.

On the other hand, regarding the operating properties of an anode, thecarbon-based anode active material induces an initial irreversiblereaction since a solid electrolyte interface (SEI) layer is formed onthe surface of the carbon-based anode active material during an initialcharge/discharge (activation) cycle. Removal (breakage) and reformationof the SEI layer through repeated charge/discharge cycles also causesdepletion of the electrolyte, thereby reducing battery capacity.

Various methods, such as formation of an SEI layer with much strongerbonding to the anode active material and formation of an oxide layer orthe like on the surface of the anode active material, have beenattempted to solve these problems. However, these methods are unsuitablefor commercialization due to problems such as deterioration inelectrical conductivity caused by the oxide layer and deterioration inproductivity caused by additional processes.

In addition, it is difficult to form an oxide layer with differentproperties on a highly non-polar anode active material and thus forminga uniform oxide layer inherently increases process cost.

Thus, there is a great need for secondary batteries capable offundamentally solving these problems.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the above andother technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies and variousexperiments to solve the above problems, the present inventors havedeveloped, as described below, an anode active material with a uniquestructure which can simultaneously solve various problems associatedwith the anode fabrication process and a lot of problems associated withoperating properties of batteries, i.e., an anode active material with astructure in which a specific composite coating layer is formed on acrystalline carbon-based core, and have also developed a novel methodfor effectively preparing such an anode active material through a simpleprocess. The present invention has been completed based on this work.

Technical Solution

In accordance with the present invention, there is provided a method forpreparing an anode active material comprising a core comprising acrystalline carbon-based material and a composite coating layercomprising at least one material selected from the group consisting oflow crystalline carbon and amorphous carbon and a hydrophilic material,the method comprising (a) mixing a precursor of a material for preparingthe at least one material selected from the group consisting of lowcrystalline carbon and amorphous carbon with the hydrophilic material,followed by purification to prepare a mixture for coating, (b) mixingthe mixture for coating with the crystalline carbon-based material toprepare a core-shell precursor in which the mixture for coating iscoated on the core comprising the crystalline carbon-based material, and(c) calcining the core-shell precursor to carbonize the material forpreparing the at least one material selected from the group consistingof low crystalline carbon and amorphous carbon into the at least onematerial selected from the group consisting of low crystalline carbonand amorphous carbon.

A crystalline carbon-based material as a core component and at least onematerial selected from the group consisting of low crystalline carbonand amorphous carbon exhibit high hydrophobicity. Generally, a materialwhich is to be converted into at least one material selected from thegroup consisting of low crystalline carbon and amorphous carbon throughcalcination (i.e., a material for preparing at least one materialselected from the group consisting of low crystalline carbon andamorphous carbon) also exhibits hydrophobicity. Therefore, if a materialfor preparing at least one material selected from the group consistingof low crystalline carbon and amorphous carbon and a hydrophilicmaterial are directly mixed with a crystalline carbon-based material,the material for preparing at least one material selected from the groupconsisting of low crystalline carbon and amorphous carbon and thehydrophilic material are difficult to homogenize, with the result thatit is difficult to obtain a desirable core-shell precursor having astructure in which a uniform mixture of such materials is coated on acore comprising a crystalline carbon-based material. Therefore, a domainwhose main component is at least one material selected from the groupconsisting of low crystalline carbon and amorphous carbon and a domainwhose main component is a hydrophilic material are formed on a compositecoating layer of an anode active material obtained through calcination,with the result that it is difficult to achieve all of the desiredeffects of the present invention.

On the other hand, according to the present invention, a precursor ofthe material for preparing at least one material selected from the groupconsisting of low crystalline carbon and amorphous carbon is mixed witha hydrophilic material, followed by purification to prepare a mixturefor coating, and the mixture for coating is then mixed with acrystalline carbon-based material. Thus is obtained a core-shellprecursor in which a uniform mixture for coating, comprising a materialfor preparing at least one material selected from the group consistingof low crystalline carbon and amorphous carbon and a hydrophilicmaterial, is coated on a core comprising a crystalline carbon-basedmaterial.

When the core-shell precursor obtained in this manner is calcined, it ispossible to obtain an anode active material having a unique structure inwhich a composite coating layer, which comprises a matrix comprising onecomponent selected from at least one material selected from the groupconsisting of low crystalline carbon and amorphous carbon and ahydrophilic material and a filler comprising a remaining componentselected therefrom, the filler being incorporated in the matrix, coversa core comprising a crystalline carbon-based material.

Generally, a carbon-based material is classified into graphite having acomplete layered crystal structure such as natural graphite, soft carbonhaving a low-crystalline layered crystal structure (graphene structurein which hexagonal carbon units are arrayed in a honeycomb shaped layerform), and hard carbon having a structure in which such structures aremixed with non-crystalline parts.

In a preferred embodiment, the crystalline carbon-based material as acore component of the present invention may be graphite or a mixture ofgraphite and low crystalline carbon and one of the components of thecomposite coating layer may be low-crystalline carbon, amorphous carbon,or a mixture thereof.

A preferred example of the precursor of the material for preparing theat least one material selected from the group consisting of lowcrystalline carbon and amorphous carbon is a pitch solution. Generally,pitch is classified into petroleum-based pitch and coal-based pitch.Therefore, the precursor may be derived from a petroleum-based material,a coal-based material, or a mixture of petroleum and coal-basedmaterials. For example, petroleum-based pitch is obtained by purifying ahigh-boiling residue remaining after crude oil is refined. Therefore, ahighly uniform coating mixture can be obtained by mixing the pitchsolution with a hydrophilic material, followed by purification.

Specifically, the purification process of the pitch solution includesthe processes of adding some materials to the pitch solution andremoving relatively low boiling impurities such as hydrocarbons andsulfur therefrom through heat treatment under an inert atmosphere at atemperature ranging from 350 to 700° C., followed by cooling andgrinding. The coating mixture may be obtained through these processes.

Especially, when the hydrophilic material is added in the pitch solutionstep, uniform dispersion of the hydrophilic material can beadvantageously maximized, as compared to when the hydrophilic materialis simply mixed with pitch.

A solution in various phases may be used as the pitch solution. Forexample, not only a low-viscosity liquid-phase solution but also ahigh-viscosity and substantially solid-phase solution may be used as thepitch solution. Of course, a solution partially containing solidcomponents may be used as the pitch solution as appropriate.

The type of the hydrophilic material as another component of thecomposite coating layer in the present invention is not particularlylimited so long as the hydrophilic material does not have negativeeffects upon operating properties of batteries while exhibiting highhydrophilicity and polarity relative to the at least one materialselected from the group consisting of low crystalline carbon andamorphous carbon. The hydrophilic material is preferably an oxide, anitride, a carbide, or the like, that does not react with lithium. Thesematerials may be used singly or as a mixture of two or more thereof.

Preferred examples of the oxide include, but are not limited to,aluminum oxide, magnesium oxide, zirconium oxide, or a mixture of two ormore thereof.

Preferred examples of the nitride include, but are not limited to,silicon nitride.

Preferred examples of the carbide include, but are not limited to,silicon carbide, cobalt carbide, or a mixture thereof.

In the present invention, the structure of the composite coating layermay be determined depending on components forming the matrix and thefiller.

In a first exemplary structure, a filler comprising a hydrophilicmaterial is incorporated in a matrix comprising at least one materialselected from the group consisting of low crystalline carbon andamorphous carbon.

In a second exemplary structure, a filler comprising at least onematerial selected from the group consisting of low crystalline carbonand amorphous carbon is incorporated in a matrix comprising ahydrophilic material.

In the composite coating layer, the content of the components of thematrix is not necessarily greater than the content of the components ofthe filler since the components of the matrix have continuous phaseswhile the components of the filler have independent phases.

In the composite coating layer, the content of the at least one materialselected from the group consisting of low crystalline carbon andamorphous carbon and the content of the hydrophilic material are notparticularly limited so long as the intended effects of the presentinvention (as described above) are achieved. In a preferred embodiment,in the composite coating layer that has been subjected to carbonizationin step (c), the at least one material selected from the groupconsisting of low crystalline carbon and amorphous carbon and thehydrophilic material may be mixed in a ratio from 1:9 to 9:1 on a weightbasis. Accordingly, in the step of carbonizing the material forpreparing carbon, when the carbonization yield is 50%, the material forpreparing the at least one material selected from the group consistingof low crystalline carbon and amorphous carbon and the hydrophilicmaterial may be mixed in a ratio from 2:9 to 18:1 on a weight basis inthe coating mixture of step (b).

The amount of the composite coating layer (coated on the anode activematerial) is preferably 0.5 to 20% by weight, based on the total amountof the anode active material. When the amount of the composite coatinglayer is excessively low or the thickness thereof is excessively small,disadvantageously, the effects of formation of the composite coatinglayer may not be achieved. Conversely, when the amount of the compositecoating layer is excessively high or the thickness thereof isexcessively great, disadvantageously, a desired core-composite coatinglayer structure may not be formed, thereby causing capacity reduction.

In the present invention, the core-shell precursor is calcined in step(c) to form the composite coating layer. Preferably, calcination isperformed under an inert atmosphere or an oxygen deficient atmosphere ata temperature ranging from 600 to 2000° C. Through such calcination, thematerial for preparing amorphous carbon is carbonized and converted intoamorphous carbon while the hydrophilic material is not converted. In apreferred example, the material for preparing amorphous carbon may havea carbonization yield of about 20 to 80% and the carbonization yield mayhave various values according to the constitution of the material forpreparing amorphous carbon.

The present invention also provides an anode active material preparedusing the method described above.

The anode active material prepared using the method described above cansimultaneously solve all problems associated with the related art sincethe anode active material has a unique structure in which a compositecoating layer with a matrix/filler structure comprising at least onematerial selected from the group consisting of low crystalline carbonand amorphous carbon and a hydrophilic material is coated on a corecomprising a crystalline carbon-based material.

First, the hydrophilic material, which is included as a matrix or fillercomponent in the composite coating layer, exhibits a high affinity for ahydrophilic solvent in a slurry for anode fabrication, thereby improvingdispersion of solid components in the slurry. Accordingly, when an anodeis fabricated by applying the slurry to a current collector,distribution uniformity between components such as a binder and theanode active material can be improved and superior electrode propertiescan thus be achieved.

Uniformity improvement through the hydrophilic material can minimize adecrease in the bonding strength between the active material layer and apartial current collector which occurs on a non-uniform electrode.Basically, the hydrophilic material increases the affinity between theactive material layer and the surface of the current collector,improving the bonding strength between the active material layer and thecurrent collector, and thereby solves the problem of increase ininternal resistance caused by separation of the active material layerfrom the current collector.

Similarly, the hydrophilic material included in the composite coatinglayer imparts hydrophilicity to at least a part of the anode activematerial. This greatly reduces impregnation time of the highly polarelectrolyte in the electrode fabrication process, thereby considerablyimproving battery productivity.

Second, the hydrophilic material included in the composite coating layerpreviously forms a layer that provides the same function as an SEIhaving a strong chemical bond and forms an even stronger bond with thesurface of the anode. This reduces the amount of irreversible ionsrequired to form the SEI layer and also minimizes removal of the SEIlayer during repeated charge and discharge, ultimately improving batterylifespan.

Third, the at least one material selected from the group consisting oflow crystalline carbon and amorphous carbon included as a matrix orfiller component in the composite coating layer minimizes the problem ofdeterioration in electrical conductivity which is caused by the presenceof the hydrophilic material. In addition, in the case of a lithiumsecondary battery, growth of lithium dendrites may occur since thecrystalline carbon-based material serving as a core has a potentialclose to that of lithium. However, this growth can be inhibited sincethe surface of the crystalline carbon-based material is coated with theat least one material selected from the group consisting of lowcrystalline carbon and amorphous carbon.

The present invention also provides an anode mix comprising the anodeactive material.

The anode mix according to the present invention comprises 1 to 20% byweight of a binder, and optionally comprises 0 to 20% by weight of aconductive material, based on the total weight of the anode mix.

Examples of the binder include polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), cellulose, polyvinyl alcohol,carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose,regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymers(EPDM), sulfonated EPDM, styrene butadiene rubbers, fluoro-rubbers,various copolymers, and polymer-saponified polyvinyl alcohols.

Any conductive material may be used without particular limitation solong as suitable conductivity is provided without causing chemicalchanges in the battery. Examples of the conductive material includegraphite, carbon blacks such as acetylene black, Ketjen black, channelblack, furnace black, lamp black and thermal black, conductive fiberssuch as carbon fibers and metallic fibers, metallic powders such ascarbon fluoride powder, aluminum powder and nickel powder, conductivewhiskers such as zinc oxide and potassium titanate whiskers, conductivemetal oxides such as titanium oxide, and polyphenylene derivatives.Specific examples of commercially available conductive materials includevarious acetylene black products (available from Chevron Chemicalcompany, Denka Singapore Private Limited and Gulf Oil company), KetjenBlack EC series (available from Armak company), Vulcan XC-72 (availablefrom Cabot company) and Super P (available from Timcal company).

Where appropriate, a filler may be added as a component to inhibitexpansion of the anode. Any filler may be used without particularlimitation so long as the filler is a fibrous material that does notcause chemical changes in the battery. Examples of the filler includeolefin-based polymers such as polyethylene and polypropylene and fibrousmaterials such as glass fibers and carbon fibers.

Optionally, other components such as viscosity controllers or adhesionpromoters may be further added singly or in combination of two or morethereof.

The viscosity controller is a component that controls the viscosity ofthe electrode mix to facilitate mixing of the electrode mix andapplication of the same to a current collector, and may be added in anamount of up to 30% by weight, based on the total weight of the anodemix. Examples of the viscosity controller include, but are not limitedto, carboxymethyl cellulose and polyvinylidene fluoride. In some cases,the afore-mentioned solvent may also act as the viscosity controller.

The adhesion promoter is an auxiliary ingredient that is added toimprove adhesion of an active material to a current collector, and ispresent in an amount of not more than 10% by weight, relative to thebinder and examples thereof include oxalic acid, adipic acid, formicacid, and acrylic acid derivatives and itaconic acid derivatives.

The present invention also provides an anode for secondary batteries inwhich the anode mix is applied to a current collector.

The anode may be produced by adding an anode material containing ananode active material, a binder or the like to a solvent such as NMP toprepare a slurry and applying the slurry to an anode current collector,followed by drying and pressing.

The anode current collector is generally fabricated to a thickness of 3to 500 μm. Any anode current collector may be used without particularlimitation so long as suitable conductivity is provided without causingchemical changes in the battery. Examples of the anode current collectorinclude copper, stainless steel, aluminum, nickel, titanium, sinteredcarbon, copper or stainless steel surface-treated with carbon, nickel,titanium or silver, and aluminum-cadmium alloys. The anode currentcollector may include fine irregularities on the surface thereof so asto enhance adhesion of anode active materials. In addition, the currentcollector may be used in various forms such as a film, a sheet, a foil,a net, a porous structure, a foam and a nonwoven fabric.

The present invention also provides a secondary battery, preferably alithium secondary battery, comprising the anode.

The lithium secondary battery has a structure in which a non-aqueouselectrolyte containing a lithium salt is impregnated into an electrodeassembly comprising a cathode, an anode, and a separator interposedbetween the cathode and the anode.

For example, the cathode may be prepared by applying a cathode activematerial to a cathode current collector, followed by drying andpressing. Optionally, the cathode may further include other componentssuch as a binder or a conductive material described above in associationwith the configuration of the anode.

The cathode current collector is generally manufactured to a thicknessof 3 to 500 μm. Any cathode current collector may be used withoutparticular limitation so long as high conductivity is provided withoutcausing chemical changes in the battery. Examples of the cathode currentcollector include stainless steel, aluminum, nickel, titanium, sinteredcarbon, or aluminum or stainless steel surface-treated with carbon,nickel, titanium or silver. Similar to the anode current collector, thecathode current collector may include fine irregularities on the surfacethereof so as to enhance adhesion to the cathode active material. Inaddition, the cathode current collector may be used in various formssuch as a film, a sheet, a foil, a net, a porous structure, a foam and anonwoven fabric.

The cathode active material is a lithium transition metal oxidecomprising two or more transition metals as a substance that causeselectrochemical reaction, and examples thereof include, but are notlimited to, layered compounds such as lithium cobalt oxide (LiCoO₂) orlithium nickel oxide (LiNiO₂) substituted by one or more transitionmetals; lithium manganese oxide substituted by one or more transitionmetals; lithium nickel-based oxides represented by the formulaLiNi_(1−y)M_(y)O₂ (in which M=Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga,the lithium nickel-based oxide including at least one of the elements,and 0.01≦y≦0.7); lithium nickel cobalt manganese composite oxidesrepresented by the formulaLi_(1+z)Ni_(b)Mn_(c)Co_(1−(b+c+d))M_(d)O_((2−e))A_(e) such asLi_(1+z)Ni_(1/3)CO_(1/3)Mn_(1/3)O₂ or Li_(1+z)Ni_(0.4)Mn_(0.4)Co_(0.2)O₂(in which −0.5≦z≦0.5, 0.1≦b≦0.8, 0.1≦c≦0.8, 0≦d≦0.2, 0≦e≦0.2, b+c+d<1,M=Al, Mg, Cr, Ti, Si or Y, A=F, P or Cl); and olivine-based lithiummetal phosphates represented by the formulaLi_(1+x)M_(1−y)M′_(y)PO_(4−z)X_(z) (in which M=a transition metal,preferably, Fe, Mn, Co or Ni, M′=Al, Mg or Ti, X═F, S or N, −0.5≦x≦+0.5,0≦y≦0.5, and 0≦z≦0.1).

The binder, the conductive material and optionally added components havebeen described above in association with the anode.

The separator is interposed between the cathode and the anode. A thininsulating film having high ion permeability and mechanical strength isused as the separator. The separator typically has a pore diameter of0.01 to 10 μm and a thickness of 5 to 300 μm. For example, a sheet ornonwoven fabric made of polyethylene or glass fibers or an olefin-basedpolymer such as polypropylene, which is chemically resistant andhydrophobic, is used as the separator.

Where appropriate, a gel polymer electrolyte may be coated on theseparator in order to improve battery stability. Representative examplesof the gel polymer include polyethylene oxide, polyvinylidene fluorideand polyacrylonitrile. When a solid electrolyte such as a polymer isused as the electrolyte, the solid electrolyte may also serve as aseparator.

The lithium salt-containing non-aqueous electrolyte comprises anon-aqueous electrolyte and a lithium salt.

Examples of the non-aqueous electrolyte include aprotic organic solventssuch as N-methyl-2-pyrollidinone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,ethylmethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane,1,2-diethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran,dimethylsulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxene, diethylether,formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane,methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate.

Examples of the non-aqueous electrolyte include organic solidelectrolytes such as polyethylene derivatives, polyethylene oxidederivatives, polypropylene oxide derivatives, phosphoric acid esterpolymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol,polyvinylidene fluoride, and polymers containing ionic dissociationgroups.

Examples of the non-aqueous electrolyte include inorganic solidelectrolytes such as nitrides, halides and sulfates of lithium such asLi₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃,Li₄SiO₄, Li₄SiO₄—LiI—LiOH and Li₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in thenon-aqueous electrolyte and may include, for example, LiCl, LiBr, LiI,LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiBF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆,LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, LiSCN, LiC(CF₃SO₂)₃, (CF₃SO₂)₂NLi,chloroborane lithium, lower aliphatic carboxylic acid lithium, lithiumtetraphenylborate and imide.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further include halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may additionally include carbon dioxide gas and may furthercontain fluoro-ethylene carbonate (FEC), propene sultone (PRS) and thelike.

In a preferred embodiment, the lithium salt-containing non-aqueouselectrolyte may be prepared by adding a lithium salt, such as LiPF₆,LiClO₄, LiBF₄ or LiN(SO₂CF₃)₂, to a mixed solvent of a cyclic carbonatesuch as EC or PC as a highly dielectric solvent and linear carbonatesuch as DEC, DMC or EMC as a low-viscosity solvent.

The present invention also provides a middle or large-sized battery packthat uses the secondary battery as a unit cell.

The middle or large-sized battery pack has a considerably large batterycell (unit cell) size, as compared to a small battery pack, in order toobtain high capacity and thus the electrolyte impregnation process orthe like requires much more time. An anode comprising a hydrophilicmaterial according to the present invention is highly desirable sinceimpregnation time can be greatly reduced.

Preferred examples of the battery pack include, but are not limited to,lithium ion secondary battery packs for vehicles or power storagedevices.

The structure of middle or large-sized battery packs using a secondarybattery as a unit cell and a fabrication method thereof are well knownin the art and a detailed explanation thereof is thus omitted in thisspecification.

BEST MODE

Now, the present invention will be described in more detail withreference to the following examples without limiting the scope of thepresent invention.

Example 1

During production of pitch having a carbonization yield of 50% as amaterial for low crystalline carbon, aluminum oxide (Al₂O₃) having amean particle diameter of about 100 nm was added, as a material havingrelatively high hydrophilicity, to a precursor of the pitch, followed bypurification to produce a pitch/aluminum oxide composite material. Here,the weight ratio of pitch to aluminum oxide was 8:2. The pitch/aluminumoxide composite material (A) and graphite (B) having a mean particlediameter of about 20 μm as a core material were homogenously mixed in aweight ratio of A:B of 10:90. This mixture was thermally treated under anitrogen atmosphere at 1200° C. for 2 hours in an electric furnace.During thermal treatment, the pitch was softened and carbonized whilebeing coated on a graphite surface in the form of a composite withaluminum oxide to produce a core-shell-structured graphite-based anodeactive material coated with a carbon/aluminum oxide composite.

The anode active material, SBR and CMC were mixed in a weight ratio ofactive material:SBR:CMC of 97.0:1.5:1.5 to prepare a slurry and theslurry was applied to a Cu-foil to prepare an electrode. The electrodewas roll-pressed to impart a porosity of about 23% and punched tofabricate a coin-type half-cell. Li metal was used as a counterelectrode in the cell and 1M LiPF₆ dissolved in a carbonate solvent wasused as an electrolyte.

Example 2

An anode active material was produced and a coin-type half-cell wasfabricated in the same manner as in Example 1, except that the weightratio of pitch to aluminum oxide (Al₂O₃) was 8:1 and the pitch/aluminumoxide composite material and the graphite were mixed in a weight ratioof 9:91.

Example 3

An anode active material was produced and a coin-type half-cell wasfabricated in the same manner as in Example 1, except that magnesiumoxide (MgO) having a mean particle diameter of about 100 nm was usedinstead of aluminum oxide (Al₂O₃).

Comparative Example 1

An anode active material was produced and a coin-type half-cell wasfabricated in the same manner as in Example 1, except that aluminumoxide was not added during pitch production and thus only pitch was usedas a coating material.

Comparative Example 2

An anode active material was produced and a coin-type half-cell wasfabricated in the same manner as in Example 1, except that the weightratio of pitch to aluminum oxide was 1:9.

Since the carbonization yield of the pitch is 50%, the content of thealuminum oxide is greater than 90% based on the total amount of thecarbon and aluminum oxide.

Comparative Example 3

During pitch production, the aluminum oxide was not added and thegraphite, pitch, and aluminum oxide were simultaneously mixed in aweight ratio of 90:8:2. This mixture was thermally treated under anitrogen atmosphere in an electric furnace in the same manner as inExample 1 to produce an anode active material and then a coin-typehalf-cell was fabricated therefrom.

Experimental Example 1

Electrolyte impregnation properties were evaluated using electrodesfabricated in accordance with Examples 1 to 3 and Comparative Examples 1to 3. The electrode was roll-pressed to impart a porosity of about 23%and the time required for 1 microliter (μl) of an electrolyte of 1MLiPF₆ dissolved in a carbonate solvent to completely permeate into thesurface of the electrode after being dropped on the surface wasmeasured. Results are shown in Table 1 below.

TABLE 1 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 Ex. 3Impregnation 88 90 86 142 95 92 time (sec)

As can be seen from Table 1, the electrodes using an anode activematerial coated with a carbon/hydrophilic material composite as inExamples 1 to 3 of the present invention exhibited considerably shortelectrolyte impregnation times, as compared to an electrode using ananode active material coated with only carbon as in ComparativeExample 1. The reason for this is that the anode active material surfacewas coated with a hydrophilic material, enabling the highly polarelectrolyte to rapidly permeate between particles.

Experimental Example 2

Charge/discharge properties were evaluated using the coin-typehalf-cells fabricated in accordance with Examples 1 to 3 and ComparativeExamples 1 to 3. Specifically, during charge, the cells were charged ina CC mode at a current density of 0.1 C to 5 mV and were then maintainedin a CV mode at 5 mV and charging was completed when current densityreached 0.01 C. During discharge, the cells were discharged in a CC modeat a current density of 0.1 C to 1.5V. As a result, the charge/dischargecapacity and efficiency of a first cycle were obtained. Then,charge/discharge was repeated 50 times under the same conditions asabove, except that the current density was changed to 0.5 C. Results areshown in Table 2 below.

TABLE 2 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 Ex. 3 Charge382.5 383.9 383.1 385.1 357.9 382.6 capacity (mAh/g) Discharge 354.1355.1 354.1 356.6 325.3 353.5 capacity (mAh/g) Efficiency (%) 92.6 92.592.6 92.6 90.9 92.4 Capacity 92 90 90 78 62 86 maintenance (%) after 50charge/discharge cycles

As can be seen from Table 2, anode active materials coated with acarbon/hydrophilic material composite as in Examples 1 to 3 of thepresent invention exhibited very high capacity maintenance after 50charge/discharge cycles, as compared to an anode active material coatedonly with carbon as in Comparative Example 1. The reason for this isthat a hydrophilic material serving the same function as SEI formed astrong bond with a core material via carbon to inhibit removal of theSEI layer during repeated charge/discharge cycles and also that amaterial having high charge/discharge voltage was coated to preventprecipitation of lithium and improve ionic conductivity.

In addition, it can be seen that anode active materials containing avery high content of a hydrophilic material that did not react withlithium as in Comparative Example 2 exhibited very low dischargecapacity and the surfaces thereof exhibited high electrical resistance,thus reducing capacity maintenance after 50 charge/discharge cycles.

It can also be seen that, when a simple mixture of graphite and ahydrophilic material was used rather than adding the hydrophilicmaterial during the pitch preparation process as in Comparative Example3, the hydrophilic material was not homogeneously distributed, insteadforming clusters. Thus, the fabricated electrode was non-uniform,thereby decreasing the capacity maintenance ratio after 50charge/discharge cycles, as compared to the examples.

INDUSTRIAL APPLICABILITY

As is apparent from the above description, advantageously, the methodaccording to the present invention can effectively produce an anodeactive material with a unique structure in that a composite coatinglayer comprising at least one material selected from the groupconsisting of low crystalline carbon and amorphous carbon and ahydrophilic material is formed on the outer surface of a core comprisinga crystalline carbon-based material with high uniformity.

It will be apparent to those skilled in the art that variousapplications and modifications can be made based on the abovedescription without departing from the scope of the invention.

The invention claimed is:
 1. A method for preparing an anode activematerial comprising a core comprising a crystalline carbon-basedmaterial and a composite coating layer comprising at least one materialselected from the group consisting of low crystalline carbon andamorphous carbon and an aluminum oxide, a magnesium oxide, a zirconiumoxide, or mixtures thereof, the method comprising: (a) mixing aprecursor of a material for preparing the at least one material selectedfrom the group consisting of low crystalline carbon and amorphous carbonwith the hydrophilic material, followed by purification to prepare amixture for coating, wherein the precursor for the raw material of theone or more materials selected from the group consisting of lowcrystalline carbon and amorphous carbon is a pitch solution and thepitch solution is mixed with an aluminum oxide, a magnesium oxide, azirconium oxide, or mixtures thereof, prior to purification of the pitchsolution; (b) homogenously mixing the mixture for coating with thecrystalline carbon-based material to prepare a core-shell precursor inwhich the mixture for coating is coated on the core comprising thecrystalline carbon-based material; and (c) calcining the core-shellprecursor to carbonize the material for preparing the at least onematerial selected from the group consisting of low crystalline carbonand amorphous carbon into amorphous carbon, wherein after mixing of thepitch solution with the aluminum oxide, the magnesium oxide, thezirconium oxide, or mixtures thereof, the mixture is subjected topurification, wherein purification comprises removing impurities bythermal treatment, followed by cooling and grinding, wherein theimpurities comprise hydrocarbons and sulfur having a relatively lowboiling point, and wherein said thermal treatment is conducted under aninert atmosphere at a temperature of 350° C. to 700° C.
 2. The methodaccording to claim 1, wherein the composite coating layer has astructure in which a filler is incorporated in a matrix comprising onecomponent selected from the at least one material selected from thegroup consisting of low crystalline carbon and amorphous carbon and thehydrophilic material, the filler comprising a remaining componentselected therefrom.
 3. The method according to claim 1, wherein thecrystalline carbon-based material comprises at least one of graphite andlow crystalline carbon.
 4. The method according to claim 1, wherein theprecursor of the material for preparing the at least one materialselected from the group consisting of low crystalline carbon andamorphous carbon is a pitch solution.
 5. The method according to claim1, wherein the precursor is derived from a coal-based material, or apetroleum-based material, or a mixture of petroleum and coal-basedmaterials.
 6. The method according to claim 1, wherein the hydrophilicmaterial comprises at least one selected from the group consisting of anoxide, a nitride, and a carbide that exhibit high hydrophilicityrelative to the at least one material selected from the group consistingof low crystalline carbon and amorphous carbon and that do not reactwith lithium.
 7. The method according to claim 6, wherein the oxidecomprises at least one selected from the group consisting of aluminumoxide, magnesium oxide, and zirconium oxide.
 8. The method according toclaim 6, wherein the nitride is silicon nitride.
 9. The method accordingto claim 6, wherein the carbide comprises at least one selected from thegroup consisting of silicon carbide and cobalt carbide.
 10. The methodaccording to claim 1, wherein the calcination is performed under aninert atmosphere or an oxygen deficient atmosphere at a temperatureranging from 600 to 2000° C.
 11. The method according to claim 1,wherein the composite coating layer has a structure in which a fillercomprising the hydrophilic material is incorporated in a matrixcomprising the at least one material selected from the group consistingof low crystalline carbon and amorphous carbon.
 12. The method accordingto claim 1, wherein the composite coating layer has a structure in whicha filler comprising the at least one material selected from the groupconsisting of low crystalline carbon and amorphous carbon isincorporated in a matrix comprising the hydrophilic material.
 13. Themethod according to claim 1, wherein, in the composite coating layerthat has been subjected to carbonization in the step (c), the at leastone material selected from the group consisting of low crystallinecarbon and amorphous carbon and the hydrophilic material are mixed in aratio from 1:9 to 9:1 on a weight basis.
 14. The method according toclaim 1, wherein the amount of the composite coating layer is 0.5 to 20%by weight, based on the total weight of the anode active material.